The present invention relates to an electric power steering (EPS) apparatus.
Conventionally, vehicle EPS apparatuses having a motor as a drive source have been known. In such an EPS apparatus, steering torque of a steering wheel transmitted to the steering system of a vehicle is detected by a steering torque sensor. Based on the detected steering torque, computation is executed to obtain a target value of assist force to be applied to the steering system to assist the operation of the steering wheel performed by a driver. The motor is actuated by receiving drive power, and the drive power is controlled in such a manner that the motor generates motor torque that corresponds to the computed target value.
However, when an abnormality occurs in the steering torque sensor, this EPS apparatus can no longer properly assist the steering operation. Accordingly, a technique has been developed in which an alternate target value of assist force is computed based on a steering angle of the steering wheel detected by using a steering angle sensor. When an abnormality occurs in the steering torque sensor, the alternate target value is used in place of the target value computed based on the steering torque.
For example, Japanese Laid-Open Patent Publication No. 2004-338562 discloses a technique in which an assist force target value is computed based on the steering angle and is given a hysteresis characteristic according to the direction in which the steering wheel is being turned. Japanese Laid-Open Patent Publication No. 2004-114755 discloses a technique in which the steering speed is multiplied by a coefficient corresponding to the steering angle to obtain an assist force target value.
However, the assist force target value computed based on the steering angle is merely an alternative to the assist force target value computed based on the steering torque. Without the steering torque sensor, which is an essential part of the feedback loop, it is difficult to continuously assist the steering operation in a stable manner. This is because disturbance such as changes in the road surface condition is likely to influence the steering operation.